Certain known electric generators normally comprise a tubular first supporting structure extending about an axis of rotation; a second supporting structure extending about the axis of rotation, substantially coaxial with the first supporting structure, and fitted to the first supporting structure to rotate about the axis of rotation; first active parts fitted to the first supporting structure; and second active parts fitted to the second supporting structure, facing the first active parts, and separated from the first active parts by an annular gap.
Known electric generators of this type are widely used on wind power turbines. More recently, permanent-magnet synchronous electric generators have also been used, particularly on direct-drive wind power turbines (i.e., comprising a blade assembly connected directly to the electric generator, with no gearboxes in between). Examples of direct-drive wind power turbines equipped with permanent-magnet synchronous electric generators are described in documents EP Patent No. 1,425,840; EP Patent No. 1,792,381; EP Patent No. 2,102,496; EP Patent No. 2,063,115; EP Patent No. 2,063,116; EP Patent No. 2,063,117; EP Patent No. 2,143,938; EP Patent No. 2,143,942; and EP Patent No. 2,143,944.
Though: (a) direct-drive wind power turbines are more efficient mechanically and cheaper to maintain than gearbox types, and (b) synchronous electric generators are more efficient electrically than asynchronous types; direct-drive wind power turbines are characterized by fairly low rotation speed, which, combined with the need for more and more electric power, makes it necessary to employ permanent-magnet synchronous electric generators with numerous poles and a high maximum torque, and therefore large-size electric generators which can pose structural problems.
The first supporting structure and first active parts define the stator or rotor of the electric generator, and the second supporting structure and second active parts define the rotor or stator, so the larger the electric generator is, the larger the first and second supporting structures are. Moreover, because it weighs on the wind power turbine structure as a whole, the weight of the electric generator must be maintained within given limits, over and above which a larger, more expensive wind power turbine is needed. Also, it is preferable that the first and second supporting structure not be too massive or heavy.
The first supporting structure is often defined by a tubular structure which, besides supporting the first active parts, also defines a load-bearing structural element of the wind power turbine as a whole, as shown, for example, in EP Patent No. 1,425,840 and EP Patent No. 2,102,496. As a result, the first supporting structure is subject to a normally small amount of strain, particularly during assembly but also possibly during operation of the electric generator. Even a small amount of strain of the first supporting structure, however, may have serious effects, by modifying the annular gap between the first and second active parts and so impairing operation of the electric generator. The normal practice, in fact, is to minimize the radial size of the annular gap to increase the efficiency of the electric generator and reduce flux dispersion, but it is often necessary to oversize the annular gap to prevent strain of the first supporting structure from affecting the electric generator.